Collagen is the principle component of bone, connective tissues, and the extracellular matrix in animals.1 The overproduction of collagen is associated with a variety of diseases, including fibrotic diseases2 and cancers.3-7 The stability of collagen relies on posttranslational modifications that occur throughout the secretory pathway.8 The most prevalent of these modifications is the hydroxylation of collagen strands by collagen prolyl 4-hydroxylases (CP4Hs), which are Fe(II)- and α-ketoglutarate (AKG)-dependent dioxygenases (FAKGDs) located in the lumen of the endoplasmic reticulum.9 Catalysis by CP4Hs convert (2S)-proline (Pro) residues in protocollagen strands into (2S,4R)-4-hydroxyproline (Hyp) residues (FIG. 1A), which are essential for the conformational stability of mature collagen triple helices.10 Importantly, CP4Hs are validated targets for treating both fibrotic diseases11 and metastatic cancer, particularly breast cancer.6 
Like all enzymes of the FAKGD superfamily, catalysis by CP4Hs requires Fe(II), and the cosubstrates AKG and dioxygen.12,39,40 The Fe(II) is bound by a conserved His-X-Asp/Glu . . . Xn . . . His motif, and AKG chelates to enzyme-bound Fe(II) using its C-1 carboxylate and C-2 keto groups, while the C-5 carboxylate group forms hydrogen bonds and engages in Coulombic interactions with a basic residue (typically arginine or lysine). All FAKGDs are believed to effect catalysis through a similar two-stage mechanism in which AKG is first oxidatively decarboxlated to generate a highly reactive Fe(IV)═O species (ferryl ion), which effects hydroxylation via a radical rebound process.12,39,40 
In vertebrates, CP4Hs are known to exist as α2β2 tetramers. In these tetramers, the α-subunit contains the catalytic and substrate-binding domains, and the β-subunit is protein disulfide isomerase, which is a multifunctional protein that is responsible for maintaining the α-subunit in a soluble and active conformation.9 Three isoforms of the α-subunit, α(I), α(II), and α(III), have been identified in humans.9 All α-subunit isoforms form tetramers with the β-subunit, which are referred to herein as the CP4H1, CP4H2, and CP4H3 holoenzymes. As the most prevalent of the isoforms, CP4H1 has been extensively characterized. Whereas the structure of the tetrameric complex is unknown, those of the individual domains of the α(I)-subunit have provided insight into the manner in which CP4Hs interact with the protocollagen substrate, as well as the means by which the α(I)-subunits dimerize to facilitate formation of the tetramer.13-16 
The development of CP4H inhibitors has been of interest since the mid 1970s. Like many FAKGDs, human CP4Hs are inhibited by simple metal chelators, such as 2,2′-bipyridine (bipy), as well as AKG mimics18, such as N-oxalyl glycine (NOG), pyridine-2,4-dicarboxylic acid (24PDC), and pyridine-2,5-dicarboxylic acid (25PDC), and 3,4-dihydroxybenzoic acid (DHB)17, as well as by simple metal chelators, such as 2,2′-bipyridine (bipy; FIG. 1B). These compounds suffer from low potency in cellular assays, insufficient selectivity for CP4H, and intolerable cytotoxicity or intolerable cytotoxicity.19,20 Still, the ethyl ester of DHB (that is, EDHB) is often used as a cellular “P4H” inhibitor,22,31 even though DHB is not selective for CP4H compared to other FAKGDs, requires high dosing, and leads to an iron-deficient phenotype.22 
The most potent inhibitors of human CP4Hs identified to date are bipyridinedicarboxylic acids (bipyDCs; FIG. 1B). Two bipyDCs have high potency: 2,2′-bipyridine-4,5′-dicarboxylic acid (bipy45′DC)38 and 2,2′-bipyridine-5,5′-dicarboxylic acid (bipy55′DC).21 Both of these bipyDCs inhibit humanCP4H competitively with respect to AKG and bind selectively to human CP4H1 compared to PHD2, another human P4H.38 An intrinsic property of bipyDCs limits their utility in a biological context. Like bipy itself, bipyDCs form tight complexes with free iron,38 which is the dominant metal in life.
With high potency and selectivity for human CP4H, the bipyDCs represent an intriguing class of compounds for the development of antifibrotic or antimetastatic therapeutics. However, these compounds possess a variety of undesirable chemical properties that have limited their development thus far. First, the bipyDCs as a class are not cell permeable, requiring the preparation of suitable cell permeable prodrugs. Second, similar to their parent bipy, the bipyDCs are capable of binding and forming complexes with free iron38 and likely other biologically relevant metals.
Thus, there is a need in the art for inhibitors of CP4H and particularly those that have high potency and selectivity for CP4H compared to other P4Hs. There is a need in the art for selective inhibitors of human CP4H. There is also a need for such inhibitors which exhibit reduced iron binding while retaining inhibitor activity and/or which are cell permeable. Inhibitors of prolyl 4-hydroxylase have been reported.19, 21, 32-34 Each of these references is incorporated by reference herein for descriptions of synthesis and structure of inhibitors, methods for synthesis of certain esters useful as prodrugs and assessment of inhibition. Compounds specifically disclosed in these references can if necessary be excluded from the claims herein.
U.S. Pat. Nos. 5,658,933 and 5,620,995 report substituted heterocyclic carboxyamide esters as ester prodrugs of prolyl hydroxylase inhibitors. U.S. Pat. No. 6,093,730 reports substituted isoquinoline-3-carboxamides as prolyl-4-hydroxylase inhibitors. These patents are incorporated by reference herein for descriptions of synthesis and structure of inhibitors and assessment of inhibition. Compounds specifically disclosed in these references can if necessary be excluded from the claims herein.